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means for detecting a drive mode of the motor vehicle, said drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, said four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between said two-wheel drive mode and said direct-coupled four-wheel drive mode;
means, in response to said drive mode detecting means, for controlling distribution of traction between the main and secondary driving wheels, said distribution controlling means comprising a changeover switch, which serves to select said drive mode, having a contact of said auto four-wheel drive mode that is positioned between contacts of said two-wheel drive mode and said direct-coupled four-wheel drive mode so as to pass said auto four-wheel drive mode when switching from said two-wheel drive mode to said direct-coupled four-wheel drive mode; and
means, in response to said distribution controlling means, for distributing traction between the main and secondary driving wheels, said traction distributing means including a friction clutch and a solenoid,
wherein said distribution controlling means comprises:
means for determining a time elapsed during switching from said auto four-wheel drive mode to said two-wheel drive mode;
means, in response to said time determining means, for continuing said auto four-wheel drive mode during a predetermined period without shifting to said two-wheel drive mode;
means for detecting turning of the motor vehicle;
means, in response to said turning detecting means, for setting a time required for switching from said four-wheel drive mode to said two-wheel drive mode; and
means, in response to said time setting means for gradually reducing engaging force of said friction clutch when carrying out switching from said four-wheel drive mode to said two-wheel drive mode.
current supply means for supplying a predetermined current to said solenoid, said current supply means being connected between one end of said solenoid and the ground; and
control circuit means for controlling said current supply means to control said predetermined current supplied to said solenoid.
monitor circuit means for monitoring an electric state between said solenoid and said control circuit means.
main and secondary driving wheels;
means for detecting a drive mode of the motor vehicle, said drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, said four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between said main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between said two-wheel drive mode and said direct-coupled four-wheel drive mode;
means, in response to said drive mode detecting means, for controlling distribution of traction between said main and secondary driving wheels, said distribution controlling means comprising a changeover switch, which serves to select said drive mode, having a contact of said auto four-wheel drive mode that is positioned between contacts of said two-wheel drive mode and said direct-coupled four-wheel drive mode so as to pass said auto four-wheel drive mode when switching from said two-wheel drive mode to said direct-coupled four-wheel drive mode; and
means, in response to said distribution controlling means, for distributing traction between said main and secondary driving wheels, said traction distributing means including a friction clutch and a solenoid,
wherein said distribution controlling means comprises:
means for determining a time elapsed during switching from said auto four-wheel drive mode to said two-wheel drive mode;
means, in response to said time determining means, for continuing said auto four-wheel drive mode during a predetermined period without shifting to said two-wheel drive mode;
means for detecting turning of the motor vehicle;
means, in response to said turning detecting means, for setting a time required for switching from said four-wheel drive mode to said two-wheel drive mode; and
means, in response to said time setting means, for gradually reducing engaging force of said friction clutch when carrying out switching from said four-wheel drive mode to said two-wheel drive mode.
current supply means for supplying a predetermined current to said solenoid, said current supply means being connected between one end of said solenoid and the ground; and
control circuit means for controlling said current supply means to control said predetermined current supplied to said solenoid.
monitor circuit means for monitoring an electric state between said solenoid and said control circuit means.
means for detecting a drive mode of the motor vehicle, the drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, the four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between the two-wheel drive mode and the direct-coupled four-wheel drive mode;
means, in response to the drive mode detecting means, for controlling distribution of traction between the main and secondary driving wheels, the distribution controlling means comprising a changeover switch, which serves to select the drive mode, having a contact of the auto four-wheel drive mode that is positioned between contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode, requiring to pass the auto four-wheel drive mode when switching from the two-wheel drive mode to the direct-coupled four-wheel drive mode; and
means, in response to the distribution controlling means, for distributing traction between the main and secondary driving wheels, the traction distributing means including a friction clutch and a solenoid,
wherein the distribution controlling means comprises:
means for determining a time elapsed during switching from the auto four-wheel drive mode to the two-wheel drive mode,
means, in response to the time determining means, for continuing the auto four-wheel drive mode during a predetermined period without shifting to the two-wheel drive mode,
means for detecting turning of the motor vehicle,
means, in response to the turning detecting means, for setting a time required for switching from the four-wheel drive mode to the two-wheel drive mode, and
means, in response to the time setting means, for gradually reducing engaging force of the friction clutch when carrying out switching from the four-wheel drive mode to the two-wheel drive mode.
main and secondary driving wheels;
means for detecting a drive mode of the motor vehicle, the drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, the four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between the two-wheel drive mode and the direct-coupled four-wheel drive mode;
means, in response to the drive mode detecting means, for controlling distribution of traction between the main and secondary driving wheels, the distribution controlling means comprising a changeover switch, which serves to select the drive mode, having a contact of the auto four-wheel drive mode that is positioned between contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode, requiring to pass the auto four-wheel drive mode when switching from the two-wheel drive mode to the direct-coupled four-wheel drive mode; and
means, in response to the distribution controlling means, for distributing traction between the main and secondary driving wheels, the traction distributing means including a friction clutch and a solenoid,
wherein the distribution controlling means comprises:
means for determining a time elapsed during switching from the auto four-wheel drive mode to the two-wheel drive mode,
means in response to the time determining means for continuing the auto four-wheel drive mode during a predetermined period without shifting to the two-wheel drive mode,
means for detecting turning of the motor vehicle,
means, in response to the turning detecting means, for setting a time required for switching from the four-wheel drive mode to the two-wheel drive mode, and
means, in response to the time setting means, for gradually reducing engaging force of the friction clutch when carrying out switching from the four-wheel drive mode to the two-wheel drive mode.
a drive mode switch arranged to detect a drive mode of the motor vehicle, the drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, the four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between the two-wheel drive mode and the direct-coupled four-wheel drive mode;
a control device connected to the drive mode switch for controlling distribution of traction between the main and secondary driving wheels, the control device comprising a changeover switch, which serves to select the drive mode, having a contact of the auto four-wheel drive mode that is positioned between contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode so as to pass the auto four-wheel drive mode when switching from the two-wheel drive mode to the direct-coupled four-wheel drive mode; and
a traction distributing device connected to the control device for distributing traction between the main and secondary driving wheels, the traction distributing device including a friction clutch and a solenoid,
wherein the control device is arranged to:
determine a time elapsed during switching from the auto four-wheel drive mode to the two-wheel drive mode;
continue, in response to the time determined, the auto four-wheel drive mode during a predetermined period without shifting to the two-wheel drive mode;
detect turning of the motor vehicle;
set, in response to the turning detected, a time required for switching from the four-wheel drive mode to the two-wheel drive mode; and
gradually reduce, in response to the time set, engaging force of the friction clutch when carrying out switching from the four-wheel drive mode to the two-wheel drive mode.
main and secondary driving wheels;
a drive mode switch arranged to detect a drive mode of the motor vehicle, the drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, the four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode where a state of the motor vehicle is automatically continuously changeable between the two-wheel drive mode and the direct-coupled four-wheel drive mode;
a control device connected to the drive mode switch for controlling distribution of traction between the main and secondary driving wheels, the control device comprising a changeover switch, which serves to select the drive mode, having a contact of the auto four-wheel drive mode that is positioned between contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode so as to pass the auto four-wheel drive mode when switching from the two-wheel drive mode to the direct-coupled four-wheel drive mode; and
a traction distributing device connected to the control device for distributing traction between the main and secondary driving wheels, the traction distributing device including a friction clutch and a solenoid,
wherein the control device is arranged to:
determine a time elapsed during switching from the auto four-wheel drive mode to the two-wheel drive mode;
continue, in response to the time determined, the auto four-wheel drive mode during a predetermined period without shifting to the two-wheel drive mode;
detect turning of the motor vehicle;
set, in response to the turning detected, a time required for switching from the four-wheel drive mode to the two-wheel drive mode; and
gradually reduce, in response to the time set, engaging force of the friction clutch when carrying out switching from the four-wheel drive mode to the two-wheel drive mode.
The present invention relates to a system for controlling a four-wheel drive for a motor vehicle.
A conventional four-wheel drive motor vehicle is shown in a Service Manual No. 629(R32-2), entitled "Introduction of Four-Wheel Drive Motor Vehicles of Nissan Skyline R32 Type", published in August, 1989. This four-wheel drive motor vehicle is constructed so that a distribution of traction between front and rear wheels corresponding to main and secondary driving wheels is changed continuously and automatically between the two-wheel drive state and the direct-coupled four-wheel drive state according to a revolution or rotating speed difference between the front and rear wheels.
Another conventional four-wheel drive motor vehicle is known which is operative in the two-wheel drive mode, auto four-wheel drive mode, and direct-coupled four-wheel drive mode which can be switched/selected stepwise. With this four-wheel drive motor vehicle, when selecting the two-wheel drive mode, a distribution of traction to the secondary driving wheels is 0%, whereas a distribution of traction to the main driving wheels is 100%, enabling cruising in the two-wheel drive state. Further, when selecting the direct-coupled four-wheel drive mode, a distribution of traction to the secondary driving wheels is 50%, and a distribution of traction to the main driving wheels is also 50%, enabling cruising in the direct-coupled four-wheel drive state. Furthermore, when selecting the auto four-wheel drive mode, a distribution of traction between the front and rear wheels corresponding to the main and secondary driving wheels is changed continuously and automatically between the two-wheel drive state and the direct-coupled four-wheel drive state according to the revolution difference between the front and rear wheels.
There is known, as a device which can change stepwise or continuously a distribution of traction between the front and rear wheels corresponding to the main and secondary driving wheels of such known four-wheel drive motor vehicles, a variable-torque clutch arranged to a traction transmission train between the front and rear wheels, which can variably control transmission torque based on variable control of an engaging force.
The variable-torque clutch currently in use is mainly of the fluid type and the electromagnetic type. In order to variably control a frictional contact force between clutch plates to obtain a controlled engaging force, the fluid variable-torque clutch carries out control of a fluid pressure to a clutch piston, whereas the electromagnetic variable-torque clutch carries out control of a current value of a proportional electromagnetic solenoid. Specifically, when selecting the two-wheel drive mode, the clutch plates are disengaged with each other; when selecting the auto four-wheel drive mode, the clutch plates come in frictional contact, producing each other with slippage and; when selecting the direct-coupled four-wheel drive mode, the clutch plates are engaged with each other without producing slippage.
However, the known four-wheel drive motor vehicles include a lever or a switch which enables switching/selecting from the two-wheel drive mode, auto four-wheel drive mode, or direct-coupled four-wheel drive mode to any one of the two. Thus, upon switching/selecting, this lever or switch provides a shock to the motor vehicle, resulting in a possible deterioration of the cruising stability thereof.
Specifically, when the motor vehicle cruising in the direct-coupled four-wheel drive mode makes a turn, the revolution difference is produced between the front and rear wheels, so that "unliberated torque" occurs in the variable-torque clutch in which the clutch plates are engaged with each other without producing slippage. And, when carrying out switching/selecting from the direct-coupled four-wheel drive mode to the two-wheel drive mode, the multiple-disc friction clutch is disengaged, so that the above "unliberated torque" accumulated in the variable-torque clutch is released suddenly, providing a shock to the motor vehicle.
It is, therefore, an object of the present invention to provide a system for controlling a four-wheel drive for a motor vehicle which contributes to an improvement of the cruising stability of the motor vehicle.
According to one aspect of the present invention, there is provided a system for controlling a four-wheel drive for a motor vehicle provide with main and secondary driving wheels, comprising:
means for detecting a drive mode of the motor vehicle, said drive mode being selected between a two-wheel drive mode and a four-wheel drive mode, said four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between the main and secondary driving wheels being in a ratio of 1:1, and an auto four-wheel drive mode wherein a state of the motor vehicle is automatically continuously changed between said two-wheel drive mode and said direct-coupled four-wheel drive mode;
means, in response to said drive mode detecting means, for controlling distribution of traction between the main and secondary driving wheels; and
means, in response to said distribution controlling means, for distributing traction between the main and secondary driving wheels, said traction distributing means including a friction clutch and a solenoid.
According to another aspect of the present invention, there is provided, in a motor vehicle:
main and secondary driving wheels;
means for detecting a drive mode of the motor vehicle, said drive mode being selectable between a two-wheel drive mode and a four-wheel drive mode, said four-wheel drive mode being selectable between a direct-coupled four-wheel drive mode with distribution of traction between said main and secondary driving wheels being in a ratio 1:1, and an auto four-wheel drive mode wherein a state of the motor vehicle is automatically continuously changed between said two-wheel drive mode and said direct-coupled four-wheel drive mode;
means, in response to said drive mode detecting means, for controlling distribution of traction between said main and secondary driving wheels; and
means, in response to said distribution controlling means, for distributing traction between said main and secondary driving wheels, said traction distributing means including a friction clutch and a solenoid.
FIG. 1 is a block diagram showing a first preferred embodiment of a system for controlling a four-wheel drive for a motor vehicle according to the present invention;
FIG. 2 is a front view showing a drive mode changeover switch;
FIG. 3 is a longitudinal section showing a transfer;
FIG. 4 is a view similar to FIG. 3, showing a high/low speed changeover device switched in the high-speed shift position;
FIG. 5 is a circuit diagram showing an oil pressure supply system;
FIG. 6 is a sectional view showing a pilot selector valve used in the oil pressure supply system;
FIG. 7 is a graph illustrating a characteristic of clutch pressure vs. duty ratio;
FIG. 8 is a view similar to FIG. 7, illustrating a characteristic of front wheel transmission torque vs. clutch pressure;
FIG. 9 is a view similar to FIG. 1, showing a controller;
FIG. 10 is view similar to FIG. 8, illustrating a characteristic of front wheel transmission torque vs. revolution difference between front and rear wheels;
FIG. 11 is flowchart showing operation of the first preferred embodiment of the present invention;
FIG. 12 is a view similar to FIG. 10, illustrating a variation in clutch pressure with the drive mode selected by the drive mode changeover switch;
FIG. 13 is a view similar to FIG. 12, illustrating a variation in clutch pressure with the drive mode selected by the drive mode changeover switch in a short time;
FIG. 14 is a view similar to FIG. 9, showing a second preferred embodiment of the present invention;
FIG. 15 is a view similar to FIG. 4, showing the transfer in FIG. 14;
FIG. 16 is a view similar to FIG. 15, showing an auxiliary transmission unit with shift sleeve moved;
FIG. 17 is a view similar to FIG. 5, showing the oil pressure supply system in FIG. 14;
FIG. 18 is a view similar to FIG. 6, showing the pilot selector valve used in the oil pressure supply system in FIG. 14;
FIG. 19 is a view similar to FIG. 14, showing the controller in FIG. 14;
FIG. 20 is a view similar to FIG. 13, illustrating a characteristic of front wheel transmission torque vs. revolution difference between front and rear wheels according to the second preferred embodiment;
FIG. 21 is a view similar to FIG. 20, illustrating a characteristic of front wheel transmission torque vs. clutch pressure according to the second preferred embodiment;
FIG. 22 is a view similar to FIG. 21, illustrating a characteristic of clutch pressure vs. duty ratio;
FIG. 23 is a view similar to FIG. 22, illustrating a characteristic of switching time vs. steering angle with vehicle speed as a parameter;
FIG. 24 is a view similar to FIG. 11, showing operation of the second preferred embodiment of the present invention;
FIG. 25 is a view similar to FIG. 19, showing a third preferred embodiment of the present invention;
FIG. 26 is a view similar to FIG. 23, illustrating a characteristic of switching time vs. lateral acceleration with a longitudinal acceleration as a parameter;
FIG. 27 is a view similar to FIG. 24, showing operation of the third preferred embodiment of the present invention;
FIG. 28 is a view similar to FIG. 25, showing a fourth preferred embodiment of the present invention;
FIG. 29 is a view similar to FIG. 16, showing the transfer in FIG. 28;
FIG. 30 is a view similar to FIG. 29, showing the auxiliary transmission unit with shift sleeve moved;
FIG. 31 is a view similar to FIG. 17, showing the oil pressure supply system in FIG. 28;
FIG. 32 is a view similar to FIG. 18, showing the pilot selector valve used in the oil pressure supply system in FIG. 31;
FIG. 33 is a view similar to FIG. 25, showing the controller in FIG. 28;
FIG. 34 is a view similar to FIG. 31, showing a two-wheel/four-wheel drive control changeover circuit;
FIG. 35 is a view similar to FIG. 27, showing operation of the fourth preferred embodiment of the present invention;
FIG. 36 is a view similar to FIG. 26, illustrating a characteristic of front wheel transmission torque vs. revolution difference between front and rear wheels according to the fourth preferred embodiment;
FIG. 37 is a view similar to FIG. 36, illustrating a characteristic of front wheel transmission torque vs. clutch pressure according to the fourth preferred embodiment;
FIG. 38 is a view similar to FIG. 37, illustrating a characteristic of clutch pressure vs. duty ratio;
FIG. 39 is a view similar to FIG. 34, showing a fifth preferred embodiment of the present invention; and
FIG. 40 is a view similar to FIG. 39, showing a sixth preferred embodiment of the present invention.
Referring to the drawings, a description will be made with regard to preferred embodiments of a four-wheel drive control system for a motor vehicle according to the present invention.
FIGS. 1 to 13 show a first embodiment of the present invention. Referring first to FIG. 1, the four-wheel drive motor vehicle is designed to cruise basically in the front-engine rear-drive (FR) mode. With this four-wheel drive motor vehicle, the following drive modes can be selected by operation of a changeover switch 87 (see FIG. 2): the two-wheel drive mode in which a distribution of traction between main driving wheels or rear wheels to secondary driving wheels or front wheels is fixed to be in the ratio 100%:0%; the auto four-wheel drive mode in which a distribution of traction between the main and secondary driving wheels is automatically set to a value according to a revolution or rotating speed difference between the two, and; the direct-coupled four-wheel drive mode in which a distribution of traction between the main and secondary driving wheels is fixed to be in the ratio 50%:50%. This embodiment also adopts an auxiliary transmission, the shift position of which being selectable by operation of a lever, not shown.
Referring to FIG. 2, the changeover switch 87 is of the rotary type, and comprises a knob 87a. Moreover, the changeover switch 87 has contacts of the two-wheel drive mode (2WD) , auto four-wheel drive mode (AUTO4), and direct-coupled four-wheel drive mode (LOCK4) arranged in this order as viewed clockwise. Switching from the direct-coupled four-wheel drive mode to the two-wheel drive mode is carried out after switching from the direct-coupled four-wheel drive mode to the auto four-wheel drive mode.
Referring again to FIG. 1, the four-wheel control system for a motor vehicle is provided with an engine 10 as a drive source, front and rear wheels 12FL-12RR, a traction transmission train 14 which can change the ratio of a traction distribution between the wheels 12FL-12RR, and a traction distribution control device 15 which serves to control a distribution of traction by the traction transmission train 14.
The traction transmission train 14 includes a transmission 20 for shifting traction out of the engine 10 according to the gear ratio selected, and a transfer 22 for dividing traction out of the transmission 20 between the front wheels 12FL, 12FR and the rear wheels or regular driving wheels 12RL, 12RR. The traction transmission train 14 is constructed so that front wheel traction divided by the transfer 22 is transmitted to the front wheels 12FL, 12FR through a front wheel output shaft 24, a front differential gear 26 and a front wheel drive shaft 28, whereas rear wheel traction is transmitted to the rear wheels 12RL, 12RR through a propeller shaft or rear wheel output shaft 30, a rear differential gear 32 and a rear wheel drive shaft 34.
Referring to FIG. 3, the transfer 22 includes in a transfer casing 40 an input shaft 42 and a first output shaft 44 disposed coaxially to butt at each other. The input shaft 42 is rotatably supported to a front casing 40a through a radial bearing 46, whereas the first output shaft 44 is rotatably supported to a rear casing 40b through a radial bearing 48, so that the two shafts 42, 44 enables relative rotation. A second output shaft 54 is rotatably supported in parallel to the input shaft 42 and the first output shaft 44 through bearings 50, 52 disposed to the front and rear casings 40a, 40b, respectively. The input shaft 42 is coupled with an output shaft 56 of the transmission 20, and the first output shaft 44 is coupled with the rear wheel output shaft 30, and the second output shaft 54 is coupled with the front wheel output shaft 24.
An auxiliary transmission unit 58 and a two-wheel/four-wheel drive changeover device 60 are arranged to the input shaft 42 and the first output shaft 44.
The auxiliary transmission unit 58 comprises a planetary gear 62, and a dog-clutch-type high/low speed changeover device 64 disposed coaxially to the planetary gear 62.
The planetary gear 62 comprises a sun gear 62a formed on the outer periphery of the input shaft 42, an internal gear 62b fixed inside the front casing 40a, a pinion gear 62c engaged with the sun gear 62a and the internal gear 62b, and a pinion carrier 62d for rotatably supporting the pinion gear 62c.
The high/low speed changeover device 64 comprises a shift sleeve 64b arranged axially slidably by spline coupling of a plurality of key grooves formed on the outer periphery of the first output shaft 44 with an internal teeth 64b 1 and having an external teeth 64b 2 arranged on the outer periphery thereof, a high-speed shift gear 64c formed on the outer periphery of the input shaft 42 which is engageable with the internal teeth 64b 1 of the shift sleeve 64b, and a low-speed shift gear 64d formed on the inner periphery of the pinion carrier 62d which is engageable with the external teeth 64b 2 of the shift sleeve 64b.
Referring to FIG. 4, when the shift sleeve 64b is slidingly moved up to a high-speed shift position H as seen in the upper disposition of the shift sleeve 64b as indicated by a fully-drawn line, the high-speed shift gear 64c and the internal teeth 64b 1 are engaged with each other. Further, when the shift sleeve 64b is slidingly moved up to a low-speed shift position L as seen in the lower disposition of the shift sleeve 64b as indicated by a fully-drawn line in FIG. 4, the low-speed shift gear 64d and the external teeth 64b 2 are engaged with each other. Furthermore, when the shift sleeve 64b is slidingly moved up to a neutral position N as seen in the lower disposition of the shift sleeve 64b as indicated by a two-dot chain line in FIG. 4, the internal teeth 64b 1 and the external teeth 64b 2 are not engaged with any of the other gears of the high/low speed changeover device 64.
Returning to FIG. 3, the two-wheel/four-wheel drive changeover device 60 comprises a wet multiple-disc friction clutch (hereafter refer to as "friction clutch") 66 for changing the ratio of a traction distribution between the front and rear wheels, a first sprocket 68 disposed rotatably to the first output shaft 44, a second sprocket 70 coupled coaxially with the second output shaft 54, and a chain 72 arranged to allow connection between the first and second sprockets 60, 70.
The friction clutch 66 comprises a clutch drum 66a coupled with the first sprocket 68, friction plates 66b spline-coupled with the clutch drum 66a, a clutch hub 66c spline-coupled with the outer periphery of the first input shaft 44, friction discs 66d coupled integrally with the clutch hub 66c, each being disposed between the friction plates 66b, a rotary member 66e disposed on the outer periphery of the first output shaft 44 and moving axially toward the clutch drum 66a to bring the friction plates 66b into contact with the friction discs 66d, a pin 66k coupled integrally with the clutch hub 66c and for engaging the clutch hub 66c with the rotary member 66e, a clutch piston 66g mounted to an inner wall of the rear casing 40b and being movable axially, a thrust bearing 66f for transmitting axial movement of the clutch piston 66g to the rotary member 66e, a cylinder chamber 66h formed between the inner walls of the clutch piston 66g and the rear casing 40b, and a return spring 66j for providing to the rotary member 66e a biasing force in the direction of the clutch piston 66g.
When an oil pressure supply system 16 provides a clutch pressure P C to an input port 74 formed through the rear casing 40b which communicates with the cylinder chamber 66h, a pressing force is generated in the cylinder chamber 66h, so that the clutch piston 66g is moved leftward as viewed in FIG. 3, which is transmitted to the rotary member 66e through the thrust bearing 66f. The friction plates 66b and friction discs 66d separated from each other come in contact with each other by movement of the friction discs 66d, producing an engaging force corresponding to the clutch pressure P C due to a frictional force. Thus, a driving force of the first output shaft 44 is transmitted, according to a predetermined ratio of a torque distribution corresponding to an engaging force of the friction clutch 66, to the second output shaft 54 through the first sprocket 68, the chain 72, and the second sprocket 70.
On the other hand, when the clutch pressure P C as supplied is decreased, and the rotary member 66e and the clutch piston 66g are moved rightward as viewed in FIG. 3 by a biasing force of the return spring 66j so that the friction plates 66b and the friction discs 66d are separated from each other, a driving force of the first output shaft 44 is not transmitted to the second output shaft 54.
A four-wheel drive gear 80 is arranged to the first sprocket 68 on the outer periphery thereof on the side of the shift sleeve 64b. When the shift sleeve 64b is moved up to the low-speed shift position L as described above in connection with FIG. 4, the external teeth 64b 2 are engaged with the low-speed shift gear 64d, and also the four-wheel drive gear 80 is engaged with the internal teeth 64b 1 . Thus, the shift sleeve 64b and the four-wheel drive gear 80 constitute a dog clutch for forcibly coupling the first output shaft 44 and the second output shaft 54 together in the low-speed shift position L.
By manual operation of an auxiliary lever, not shown, the shift sleeve 64b of the high/low speed changeover device 64 which is of the dog clutch type is slidingly moved up to the high-speed shift position H, the neutral position N, or the low-speed shift position L through a fork having a pointed end 84 as shown in FIG. 3. Referring to FIG. 4, disposed inside the front casing 40a are a high-speed shift position sensor 86 for sensing that the shift sleeve 64b is slidingly moved up to the high-speed shift position H, and a low-speed shift position sensor 88 for sensing that the shift sleeve 64b is slidingly moved up to the low-speed shift position L. A detection signal S H of the high-speed shift position sensor 86 and a detection signal S L of the low-speed shift position sensor 88 are always input to a controller 18 as will be described later.
Referring to FIG. 5, the oil pressure supply system 16 has a circuit structure as shown in FIG. 5, and provides a predetermined clutch pressure P C to the input port 74 of the transfer 22.
The oil pressure supply system 16 has as an oil pressure source a main pump 100 of the normal/reverse rotation type connected directly to and driven by the first output shaft 44, and a sub-pump 104 of the normal rotation type disposed in parallel with the main pump 100 and driven by an electric motor or sub-motor 102 as a power source. The main pump 100 and the sub-pump 104 inhale hydraulic fluid within an oil tank 105 through strainers 106a, 108a, and discharge it into ducts 106b, 108b on the discharge side. Connected to a convergent duct 110a which converges the ducts 106b, 108b is an oil element 112 to which a relief passage 116 is connected on the upstream side thereof, i.e. on the side of the main pump 100 and the sub-pump 104, the relief passage 116 having the other end connected to a lubricating system 114. Moreover, a line-pressure regulating valve 118 is connected to the oil element 112 on the downstream side thereof. Connected to ducts 110b, 110c, 110e which branch off from the convergent duct 110a are an electromagnetic selector valve 120, a clutch-pressure regulating valve 122, and a pressure reducing valve 124 on the input side thereof, respectively. Moreover, connected to the clutch-pressure regulating valve 122 on the output side thereof is a pilot selector valve 126 on the input side thereof, which supplies the clutch pressure P C to the transfer 22 when receiving a pilot pressure out of the electromagnetic selector valve 120, whereas connected to the pressure reducing valve 124 on the output side thereof is a duty-control solenoid valve 128 on the input side thereof. Arranged in the oil tank 105 are a temperature sensor 130 for sensing the temperature of hydraulic fluid, a hydraulic switch 132 for detecting a pressure reduced by the line-pressure regulating valve 118, and a pressure switch 134 for detecting the clutch pressure P C output from the pilot selector valve 126, detection signals thereof being output to the controller 18. As for the actual motor vehicles, the oil pressure supply system 16 is arranged inside the transfer 22. The main pump 100 for inhaling hydraulic fluid out of the oil tank 105 is coupled with the first output shaft 44 through first and second gears 136a, 136b as shown in FIG. 3, whereas the sub-pump 104 is coupled with the electric motor 102 mounted to the rear casing 40b on the outside thereof.
Next, referring to FIG. 5, component parts of the oil pressure supply system 16 will be described in detail.
The main pump 100 rotating in the normal direction inhales hydraulic fluid out of the oil tank 105 through the strainer 106a connected to a suction duct 106c at an end thereof, and the sub-pump 104 also inhales hydraulic fluid out of the oil tank 105 through the strainer 108a connected to a suction duct 108c at an end thereof. Check valves 106d, 108d are arranged in the discharge ducts 106b, 108b of the main pump 100 and the sub-pump 104, respectively, and a bypass passage 140 is arranged to allow communication between the discharge duct 106b of the main pump 100 and the discharge duct 108c of the sub-pump 104. The bypass passage 140 comprises a bypass duct 140a and a triple check valve 140b arranged therein, and is constructed so that when the discharge duct 106b becomes in the negative pressure state, the check valve 140b opens to form a communication passage for allowing passage of hydraulic fluid in the direction of a dotted arrow as shown in FIG. 3.
The relief passage 116 connected to the convergent duct 110a on the upstream side of the oil element 112 comprises a relief duct 116a having the other end connected to the lubricating system 114, and a double spring check valve 116b arranged therein. When an oil pressure on the upstream side of the oil element 112 becomes greater than a predetermined value due to clogging produced in a filter of the oil element 112, the check valve 116b opens to form a communication passage for allowing passage of hydraulic fluid in the direction of a dotted arrow as shown in FIG. 3.
The line-pressure regulating valve 118 comprises a pressure regulating valve of the inner pilot and spring type, including a spool arranged slidably in a cylindrical valve housing having an input port 118 A connected to the convergent duct 110a, an output port 118 B connected to the lubricating system 114 and inner pilot ports 118 P1 , 118 P2 receiving primary and secondary pressures through stationary orifices, and a return spring 118a for biasing the spool on the one end side. A supply pressure P L increased through the main pump 100 or the sub-pump 104 is decreased to a predetermined value by the line-pressure regulating valve 118, which is provided to the electromagnetic selector valve 120, the clutch-pressure regulating valve 122, and the pressure reducing valve 124. Hydraulic fluid flowing out of the output port 118 B upon decreasing is returned to the lubricating system 114.
The clutch-pressure regulating valve 122 comprises a pressure regulating valve of the inner and outer pilot and spring type, including a spool arranged slidably in a cylindrical valve housing having an input port 122 A connected to the duct 110c, an output port 122 B connected to the pilot selector valve 126, an inner pilot port 122 P1 receiving as a pilot pressure the secondary pressure through a stationary orifice and an outer pilot port 122 P2 receiving a control pressure out of the duty-control solenoid valve 128, and a return spring 122a for biasing the spool on the one end side. The clutch-pressure regulating valve 122 is constructed so that when receiving no pilot control pressure out of the duty-control solenoid valve 128, a communication passage between the input port 122 A and the output port 122 B is closed so as not to output the secondary pressure, whereas when receiving the pilot control pressure out of the duty-control solenoid valve 128, the spool is moved to output from the output port 122 B , as the clutch pressure P C , the secondary pressure corresponding to the pilot control pressure.
The pressure reducing valve 124 comprises a pressure reducing valve of the inner pilot and spring type and with a constant secondary pressure, including a spool arranged slidably in a cylindrical valve housing having an input port 124 A connected to the duct 110e, an output port 124 B connected to the duty-control solenoid valve 128, an inner pilot port 124 P receiving as a pilot pressure the secondary pressure out of the output port 124 B through a stationary orifice and a drain port 124 H , and a return spring 124a for biasing the spool on the one end side. When the spool is moved to a predetermined position by the pilot pressure supplied to the inner pilot port 124 P , the primary pressure out of the input port 124 A is supplied, as a control pressure having a predetermined reduced value, to the duty-control solenoid valve 128.
The duty-control solenoid valve 128 has three ports and two positions. This valve includes an input port 128 A connected to the pressure reducing valve 124, a drain port 128 R connected to a drain, an output port 128 B connected to an outer pilot port 122 P2 of the clutch-pressure regulating valve 122, and a return spring 127a. The duty-control solenoid valve 128 is movable between a normal position 128b wherein a spool arranged in the valve allows communication between the output port 128 B and the drain port 128 R , and an operating position 128c wherein the spool allows communication between the input port 128 A and the output port 128 B . When the controller 18 provides to a solenoid 128d an exciting current i 0 with a predetermined duty ratio, the spool is moved from the normal position 128b to the operating position 128c against the return spring 128a during a period of time that the current i 0 is turned on, outputting to the clutch-pressure regulating valve 122 the pilot control pressure corresponding to the duty ratio. Therefore, when the duty-control solenoid valve 128 supplies the control pressure to the outer pilot port 122 P2 , the clutch-pressure regulating valve 122 provides the clutch pressure P C corresponding to the pilot control pressure, so that an engaging force of the friction clutch 66 is controlled according to this, obtaining a distribution of drive torque to the front wheels in accordance with the clutch pressure P C .
The electromagnetic selector valve 120 is of the spring-offset type, and has three ports and two positions. This valve includes an input port 120 A receiving a line pressure, an output port 120 B connected to an outer pilot port 126 P1 of the pilot selector valve 126, and a drain port 120 D . The electromagnetic selector valve 120 is movable between a normal position 120b wherein a spool arranged in the valve closes the input port 120 A and allows communication of the output port 120 B with the drain port 120 D , and an operating position 120c wherein the spool allows communication between the input port 120 A and the output port 120 B and closes the drain port 120 D . When the controller 18 provides to a solenoid 120d an exciting current i 1 , the spool is moved to the operating position 120c against the return spring 120a during a period of time that the current i 1 is turned on, providing the pilot control pressure to the outer pilot port 126 P1 , of the pilot selector valve 126. On the other hand, when the exciting current i 1 out of the controller 18 is turned off, the spool is returned to the normal position 120b by a pressing force of the return spring 120a, so that the pilot control pressure being supplied to the outer pilot port 126 P1 , is removed through the drain port 120 D .
Referring also to FIG. 6, the pilot selector valve 126 includes a spool 126e arranged slidably in a cylindrical housing 126i having an input port 126 A receiving the secondary pressure out of the clutch-pressure regulating valve 122, an output port 126 B providing the secondary pressure to the transfer 22, an outer pilot port 126 P1 receiving the pilot control pressure when the solenoid 120d of the electromagnetic selector valve 120 is turned on and a drain port 126 H , and a return spring 126a for biasing the spool 126e on the one end side.
When supplying no pilot control pressure to the outer pilot port 126 P1 , the spool 126e of the pilot selector valve 126 is moved to a two-wheel drive (2WD) mode position 126b wherein the input and output ports 126 A , 126 B are closed, and the output port 126 B is in communication with the drain port 126 D as seen in a left half in FIG. 6. On the other hand, when the solenoid 120d of the electromagnetic selector valve 120 is turned on, the spool of the electromagnetic selector valve 120 is moved to a four-wheel drive (4WD) mode position 126c wherein the spool is placed in the operating position 120c to supply the pilot control pressure to the outer pilot port 126 P1 , and the input port 126 A is in communication with the output port 126 B as seen in a right half in FIG. 6.
In such a way, the pilot selector valve 126 is driven by the pilot control pressure out of the electromagnetic selector valve 120, i.e. the spool 126e is driven by the pilot control pressure having a high value, so that even when the spool 126e has a great slide resistance due to dust, chips, etc. attached to a slide passage thereof, sliding of the spool 120e can be ensured.
Referring to FIG. 7, a characteristic view shows a correlation between the clutch pressure P C of the clutch-pressure regulating valve 122 and a duty ratio D of the exciting current i 0 supplied to the solenoid 128d of the duty-control solenoid valve 128, the clutch pressure P C being nonlinearly parabolically increased in accordance with an increase in the duty ratio D. According to the clutch pressure P C which the oil pressure supply system 16 supplies to the friction clutch 66, a predetermined frictional force is produced between the friction plate 66b and the friction disc 66d, which causes an engaging force of the friction clutch 66. According to this, drive torque is dividedly transmitted to the front and rear wheels. Referring to FIG. 8, a characteristic view shows a correlation between the pressure P C supplied to the friction clutch 66 and a torque T 2 transmitted to the front wheels. As seen from FIG. 8, the torque T 2 transmitted to the front wheels is linearly varied in accordance with the pressure P C supplied to the friction clutch 66. That is, with the transfer 22, the ratio of a torque distribution between the front and rear wheels can be changed in accordance with the duty ratio D of the exciting current i 0 continuously, i.e. in the range from 0%:100% to 50%:50%. Concretely, the ratio is 0% to 100% when a value of the exciting current i 0 is zero, or the exciting current i 0 itself is not supplied, and it is 50% to 50% when the duty ratio D of the exciting current i 0 is equal to the maximum set value.
Returning to FIG. 1, the traction distribution control device 15 comprises front and rear wheel revolution sensors 17F, 17R, the above high-speed and low-speed shift position sensors 86, 88, a drive mode switch 90 for detecting the drive mode selected by the changeover switch 87, and serves to output the exciting currents i 0 , i 1 to the oil pressure supply system 16 in accordance with detection signals of these sensors. In the first embodiment, the controller 18 is constructed to also carry out control for enabling the oil pressure supply system 16 to keep a predetermined oil pressure, and it is thus provided with the above oil temperature sensor 130 and hydraulic switches 132, 134, and outputs a motor control signal S M to the oil pressure supply system 16 in accordance with the detection signals of these sensors.
The front and rear wheel revolution sensors 17F, 17R are arranged to the front wheel output shaft 24 and the rear wheel propeller shaft 30 in predetermined positions thereof, respectively, and are constructed to detect revolutions of the shafts optically or electromagnetically so as to output to the controller 18 circumferential speeds of the wheel or wheel speeds as front and rear wheel revolution detection values nF, nR in the form of a pulse signal or a sine wave signal, respectively. The front and rear wheel revolution sensors 17F, 17R may be of the type as disclosed in JP-A 1-195126.
The drive mode switch 90 serves to output a drive mode M selected by the rotary changeover switch 87. The drive mode switch 90 outputs a signal indicative of M=2 when the drive mode as selected corresponds to the two-wheel drive mode, a signal indicative of M=AUTO4 when the drive mode as selected corresponds to the auto four-wheel drive mode or full-time four-wheel mode, and a signal indicative of M=LOCK4 when the drive mode as selected corresponds to the direct-coupled four-wheel drive mode.
Referring to FIG. 9, the controller 18 is provided with a microcomputer 7 for carrying out traction distribution control, a microcomputer 8 for carrying out the above control of keeping a predetermined oil pressure, a drive circuit 31a for supplying in accordance with a control signal CS 0 out of the microcomputer 7 the exciting current i 0 having a predetermined duty ratio to the solenoid 128d of the duty-control solenoid valve 128 in the oil pressure supply system 16, a drive circuit 31b for supplying the exciting current i 1 which is turned on and off in accordance with a control signal CS 1 out of the microcomputer 7 to the solenoid 120d of the electromagnetic selector valve 120 in the oil pressure supply system 16, and a motor drive circuit 103 for carrying out chopper control of the electric motor 102 in accordance with the motor control signal S M out of the microcomputer 8 so as to obtain a revolution or rotating speed in accordance therewith.
The microcomputer 7 is provided with an input interface circuit 7a having the analog to digital (A/D) conversion function for reading as detection values the detection signals of the sensors 86, 88, 90, 17F, 17R, a processing unit 7b for carrying out computing/processing for traction distribution control according to a predetermined program (see FIG. 11), a storage unit 7c such as a read-only memory (ROM), a random access memory (RAM) or the like, and an output interface circuit 7d for outputting the control signal CS 0 of the duty ratio D for commanding the clutch pressure P C which determines a distribution of torque to the front wheels obtained by the processing unit 7b, and the control signal CS 1 for determining whether to output the clutch pressure P C or not. On the other hand, the microcomputer 8 is provided with an input interface circuit 8a having the A/D conversion function for reading as detection values the detection signals of the sensors 130, 132, 134, a processing unit 8b, a storage unit 8c such as a ROM, a RAM or the like, and an output interface circuit 8d having the digital to analog (D/A) conversion function for outputting, as the analog voltage signal S M , for example, a sub-motor revolution command value obtained by the processing unit 8b.
Referring to FIG. 11, in accordance with the mode signal M out of the drive mode switch 90, the high-speed shift position detection signal S H out of the high-speed shift position sensor 86, the low-speed shift position detection signal S L out of the low-speed shift position sensor 88, the front wheel revolution detection value nF out of the front wheel revolution sensor 17F, and the rear wheel revolution detection value nR out of the rear wheel revolution sensor 17R, the microcomputer 7 determines the front wheel torque distribution command value T 2 , and calculates the duty ratio D for determining the clutch pressure P C corresponding to the command value T 2 , producing the control signal CS 0 having a command value corresponding to the duty ratio D. The microcomputer 7 also controls the control signal CS 1 , in the on or off state. These control signals CS 0 , CS 1 are output to the drive circuits 31a, 31b, respectively.
The drive circuit 31a is provided with a pulse duration modulation circuit, for example, for outputting an exciting current of the duty ratio D in accordance with a command value of the control signal CS 0 which is in the form of an analog voltage signal output from the microcomputer 7, and serves to output to the solenoid 128d of the duty-control solenoid valve 128 the exciting current i 0 of the duty ratio D in accordance with the command value of the control signal CS 0 .
The drive circuit 31b serves to convert the control signal CS 1 out of the microcomputer 7 into the exciting current i 1 having an enough value to excite the solenoid 120d of the electromagnetic selector valve 120, which is output to the solenoid 120d of the electromagnetic selector valve 120.
Moreover, in the first embodiment, the controller 18 carries out processing, i.e. control for enabling the oil pressure supply system 16 to supply a predetermined oil pressure, as follows. When the hydraulic switch 132 detects, for example, that the line pressure P L downstream of the oil element 112 of the convergent duct 110a is lower than a set value in accordance with a control program, not shown, the control signal S M indicative of a revolution command value determined in accordance with the oil temperature detection value S Y of the oil temperature sensor 130 is calculated to control a discharge pressure or discharged oil amount of the sub-pump 104, which is provided to the motor drive circuit 103 to control a revolution or rotating speed of the sub-motor 104, thus maintaining the line pressure P L out of the oil pressure supply system 16 at a predetermined value.
Next, a description will be made with regard to processing executed by the microcomputer 7 of the controller 18, i.e. a fundamental principle on traction distribution control.
In the first embodiment, as described above, the changeover switch 87 ensures switching among the three drive modes: the two-wheel drive mode, the auto four-wheel drive mode, and the direct-coupled four-wheel drive mode. When selecting the two-wheel drive mode, a distribution of traction between the rear and front wheels is fixed to be in the ratio 100%:0%. When selecting the auto four-wheel drive mode, it is automatically set to a value according to the revolution or rotating speed difference between the two wheels. And, when selecting the direct-coupled four-wheel drive mode, it is fixed to be in the ratio 50%:50%.
Concretely, when selecting the auto four-wheel drive mode, a revolution or rotating speed difference ΔV W between the front and rear wheels is calculated in accordance with the following formula: ΔV W =nR-nF (1)
That is, the revolution difference ΔV W is obtained by subtracting an average front wheel speed or front wheel revolution detection value nF obtained out of an average revolution of the secondary or front driving wheels 12FL, 12FR from an average rear wheel speed or rear wheel revolution detection value nR obtained out of an average revolution of the main or rear driving wheels 12RL, 12RR. In accordance with the revolution difference ΔV W as calculated, the front wheel torque distribution command value T 2 is determined by using a characteristic as shown in FIG. 10.
Moreover, as described above, the changeover switch 87 has the contact of the auto four-wheel drive mode positioned between the contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode. Switching between the two-wheel drive mode and the direct-coupled four-wheel drive mode is carried out after putting a distribution of traction in the auto four-wheel drive state. If the revolution difference is produced between the front and rear wheels due to turning of the motor vehicle in the direct-coupled four-wheel drive state, and "unliberated torque" occurs in the two-wheel/four-wheel drive changeover device 60, switching of the changeover switch 87 from the direct-coupled four-wheel drive mode to the two-wheel drive mode in a short time causes sudden release of "unliberated torque" accumulated in the two-wheel/four-wheel drive changeover device 60, resulting in a possible occurrence of a shock in the motor vehicle. Thus, according to traction distribution control of the present invention, it is determined whether or not switching for the direct-coupled four-wheel drive mode to the two-wheel drive mode is carried out in a short time. If switching is carried out in a short time, a distribution of traction in the auto four-wheel drive state is carried out without shifting to the two-wheel drive mode until a predetermined time t 2 elapses. The predetermined time t 1 corresponds to a switching determination time t 1 used in determination of traction distribution control. The friction plate 66b and the friction disc 66d come in frictional contact with each other with slippage produced during the switching determination time t 1 , absorbing the revolution difference between the front and rear wheels.
Referring next to FIG. 11, a description will be made with regard to processing of four-wheel drive control based on the above fundamental principle. This processing is a timer interrupt processing executed every predetermined period ΔT S , e.g. 10 msec. In this processing, a timer, which virtually comprises a counter having a count value n 1 , serves to measure the switching determination time t 1 . A time calculated by n 1 ΔT S is compared with the switching determination time t 1 .
At a step S10, it is determined that the high-speed shift position detection signal S H is output from the high-speed shift position sensor 86, and the low-speed shift position detection signal S L is not output from the low-speed shift position sensor 88. If the high-speed shift position detection signal S H is on, and the low-speed shift position detection signal S L is off, control proceeds to a step S11, whereas if not, i.e. the high-speed shift position detection signal S H is off, and the low-speed shift position detection signal S L is on, control returns to a main program.
At the step S11, the drive mode detection value M is read out of the drive mode switch 90. At a subsequent step S12, it is determined whether or not the drive mode detection value M read at the step S11 corresponds to 2 indicative of the two-wheel drive mode. If M=2, control proceeds to a step S13, whereas if not, control proceeds to a step S24.
At the step S13, the latest previous drive mode value M 0 is read, which is stored in the RAM of the storage unit 7c, then, control proceeds to a step S14. At the step S14, it is determined whether or not the previous value M 0 of the latest drive mode read at the step S13 corresponds to AUTO4 indicative of the auto four-wheel drive mode. If M 0 =AUTO4, control proceeds to a step 16, whereas if not, control proceeds to a step S21.
At the step S16 an elapsed time during switching of the changeover switch 87 from the direct-coupled four-wheel mode to the two-wheel drive mode is calculated by multiplying the count value n 1 of the counter by the sampling time ΔT S . And, it is determined whether or not the elapsed time as calculated is greater than a predetermined value or switching determination time. If t 1 . If n 1 ΔT S ≥t 1 , control proceeds to a step S21, whereas if not, control proceeds to a step S17.
At the step S17, the front and rear wheel revolution detection values nF, nR are read out of the front and rear wheel revolution sensors 17F, 17R, respectively. At a subsequent step S18, using the front and rear wheel revolution detection values nF, nR read at the step S17, the revolution difference ΔV W between the front and rear wheels is calculated according to the formula (1). At a subsequent step S19, the front wheel torque distribution command value T 2 is set in accordance with the revolution difference ΔV W between the front and rear wheels calculated at the step S18, and the characteristic as shown in FIG. 10, then, control proceeds to a step S20.
At the step S21, the counter is reset to zero. At a subsequent step S22, the front wheel torque distribution command value T 2 is set to zero, then, control proceeds to a step S23. At the step S23, the drive mode detection value M read at the step S11 is stored as the latest previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S20.
At the step S24, it is determined whether or not the drive mode detection value M read at the step S12 corresponds to AUTO4 indicative of the auto four-wheel drive mode. If M=AUTO4, control proceeds to a step S24A, whereas if not, control proceeds to a step S26.
At the step S24A, 1 (one) is added to the count value n 1 of the counter, then, control proceeds to a step S25. At the step S25, the drive mode detection value M read at the step S11 is stored as the previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S17.
At the step S26, the front wheel torque distribution command value T 2 is set to 50. At a subsequent step S27, the drive mode detection value M read at the step S11 is stored as the previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S20.
At the step S20, the front wheel torque distribution command value T 2 set at any of the steps S19, S22, S26 is output, then, control returns to the main program.
It is noted that the steps S16, S24A correspond to time determining means, and the steps S16 to S19 correspond to auto four-wheel drive mode continuing means.
The front wheel torque distribution command value T 2 set in such a way is subjected to D/A conversion in the output interface circuit 7d of the microcomputer 7. Thus, the control signals CS 0 , CS 1 having analog voltage values are input to the drive circuits 31a, 31b. The drive circuit 31a outputs the exciting current of the duty ratio D to the solenoid 128d of the duty-control solenoid valve 128 in the oil pressure supply system 16 in accordance with a command value of the control signal CS 0 , whereas the drive circuit 31b outputs the exciting current of the duty ratio D to the solenoid 120d of the electromagnetic selector valve 120 in the oil pressure supply system 16 in accordance with a command value of the control signal CS 1 .
As a result, when T 2 ≠0, the electromagnetic selector valve 120 is such that the input port 120 A is in communication with the output port 120 B , so that the pilot selector valve 126 becomes in the state as shown in the right half in FIG. 6 wherein a pressure regulated by the clutch-pressure regulating valve 122 is supplied to the outer pilot port 126 P1 of the pilot selector valve 126, and can be thus supplied to the friction clutch 66. In that case, with the duty-control solenoid valve 128, the spool is moved from the first or normal position 128b to the second or operating position 128c against the return spring 128a so as to output the exciting current i 0 of a predetermined duty ratio to the pilot port 122 P2 of the clutch-pressure regulating valve 122, so that the pressure regulated by the clutch-pressure regulating valve 122, i.e. the pressure P C supplied to the clutch in accordance with the front wheel torque distribution command value T 2 , is controlled to a predetermined value, which is supplied to the friction clutch 66 through the pilot selector valve 126. The clutch pressure P C is supplied to the input port 74 of the transfer 22 from the oil pressure supply system 16. In accordance with the clutch pressure P C as supplied, the friction plate 66b and the friction disc 66d come in frictional contact with each other. A driving force corresponding to this frictional contact force serves to drive the clutch hub 66c of the friction clutch 66, which is transmitted to the front wheel output shaft 24 through the chain 72, and further to the front wheels 12FL, 12FR through the traction transmission train 14. Thus, traction transmitted to the rear wheels 12RL, 12RR is reduced by a part corresponding to this, achieving the auto four-wheel drive state or the direct-coupled four-wheel drive state, each having a predetermined distribution of traction set by processing as shown in FIG. 11.
On the other hand, when T 2 =0, the control signal CS 1 is not output, so that the electromagnetic selector valve 120 is such that the input port 120 A is not in communication with the output port 120 B . The pilot selector valve 120 becomes in the state as shown in the left half in FIG. 6 wherein the pressure regulated by the clutch-pressure regulating valve 122 is not supplied to the outer pilot port 126 P1 of the pilot selector valve 126, and cannot be thus supplied to the friction clutch 66. In that case, since the control signal corresponding to the exciting current i 0 (=0) is input to the duty-control solenoid valve 128, the spool is stopped in the first position 128b, or moved from the second position 128c to the first position 128b, having zero control pressure. Thus, the pressure regulated by the clutch-pressure regulating valve 122 also is not controlled to a value corresponding to the front wheel torque distribution command value T 2 . In that case, the clutch pressure P C is zero. Therefore, an oil pressure is not supplied to the input port 74 of the transfer 22 from the oil pressure supply system 16, so that the friction plate 66b and the friction disc 66d do not come in frictional contact with each other. Traction is not thus transmitted to the front wheel output shaft 24, obtaining the two-wheel drive state.
Next, operation of this embodiment in connection with FIG. 11 will be described.
Suppose a case that when the four-wheel drive motor vehicle cruising in the direct-coupled four-wheel drive mode makes a turn, and produces the revolution difference between the front and rear wheels, a driver switches the changeover switch 87 to the auto four-wheel drive mode, then to the two-wheel drive mode. In that case, suppose that the elapsed time n 1 ΔT S during switching from the auto four-wheel mode to the two-wheel drive mode is greater than the switching determination time t 1 .
First, when switching the changeover switch 87 from the direct-coupled four-wheel drive mode to the auto four-wheel drive mode, at the step S10, the high-speed shift position detection signal S H is output from the high-speed shift position sensor 86, whereas the low-speed shift position detection signal S L is not output from the low-speed shift position sensor 88, so that control proceeds to the step S11 where the drive mode detection value M is read out of the drive mode switch 90. At the step S12, due to the drive mode detection value M=AUTO4 (auto four-wheel drive mode), control proceeds to the step S24. At the step S24, due to the drive mode detection value M=AUTO4, control proceeds to the step S24A where 1 (one) is added to the count value n 1 of the counter. At the subsequent step S25, the current drive mode detection value M (=AUTO4 ) is stored as the previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S17. At the step S17, the front and rear wheel revolution detection values nF, nR from the front and rear wheel revolution sensors 17F, 17R, respectively, and at the subsequent step S18, the revolution difference ΔV W between the front and rear wheels is calculated using the front and rear wheel revolution detection values nF, nR. Then, at the step S19, the front wheel torque distribution command value T 2 is set in accordance with the revolution difference ΔV W between the front and rear wheels calculated at the step S18, and the characteristic as shown in FIG. 10, then, control proceeds to the step S20 where the front wheel torque distribution command value T 2 is output.
Second, when switching the changeover switch 87 from the auto four-wheel drive mode to the two-wheel drive mode, control proceeds from the step S10 to the step S11 where the drive mode detection value M is read out of the drive mode switch 90. At the step S12, due to the drive mode detection value M=2 (two-wheel drive mode), control proceeds to the step S14. At the step S14, due to the previous drive mode value M 2 =AUTO4, control proceeds to the step S16. At the step S16, since the elapsed time n 1 ΔT S during switching from the auto four-wheel mode to the two-wheel drive mode is greater than the switching determination time t 1 , the condition of n 1 ΔT S ≥t 1 is satisfied, so that control proceeds to the step S21. At the step S21, the count value n 1 of the counter is reset to zero, and at the subsequent step S22, the front wheel torque distribution command value T 2 is set to zero. At the step S23, the current drive mode detection value M (=2) is stored as the previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S20 where the front wheel torque distribution command value T 2 (=0) is output.
As a result, when carrying out switching from the direct-coupled four-wheel drive mode (LOCK4) to the auto four-wheel drive mode (AUTO4), a variation in the clutch pressure P C is decreased gradually as shown in FIG. 12 due to a response lag of the friction clutch 66 and a filtering of command torque, and the clutch pressure P C comes to the front wheel torque distribution command value T 2 in accordance with the revolution difference ΔV W between the front and rear wheels calculated at the step S18. The clutch pressure P C is lowered gradually in such a way, so that even if the revolution difference between the front and rear wheels is produced by making a turn in the direct-coupled four-wheel drive mode, and "unliberated torque" occurs in the two-wheel/four-wheel drive changeover device 60, the friction plate 66b and the friction disc 66d cooperate with each other with slippage produced to absorb the revolution difference between the front and rear wheels. This reduces "unliberated torque" accumulated in the two-wheel/four-wheel drive changeover device 60, resulting in no occurrence of a shock in the motor vehicle even upon switching from the direct-coupled four-wheel drive mode to the auto four-wheel drive mode.
Third, when carrying out switching from the auto four-wheel drive mode (AUTO4) to the two-wheel drive mode (2WD), a variation in the clutch pressure P C is small as shown in FIG. 12, so that even if "unliberated torque" is accumulated in the two-wheel/four-wheel drive changeover device 60, no shock occurs during mode switching.
Next, suppose a case that when the four-wheel drive motor vehicle cruising in the direct-coupled four-wheel drive mode makes a turn, and produces the revolution difference between the front and rear wheels, the driver switches in a short time the changeover switch 87 from the direct-coupled four-wheel drive mode to the two-wheel drive mode. It is noted that switching in a short time means an operation that the elapsed time n 1 ΔV W during switching from the direct-coupled four-wheel drive mode to the two-wheel drive mode is largely smaller than the switching time t 1 .
First, the auto four-wheel drive mode is achieved in the middle of switching in a short time from the direct-coupled four-wheel drive mode to the two-wheel drive mode, so that when control proceeds from the step S10 to the step S11, the drive mode detection value M (=AUTO4) is read out of the drive mode switch 90. At the step S12, due to the drive mode detection value M=AUTO4, control proceeds to the step S24. At the step S24, due to the drive mode detection value M=AUTO4, control proceeds to the step S24A where 1 (one) is added to the count value n 1 of the counter. At the subsequent step S25, the current drive mode detection value M (=AUTO4) is stored as the previous drive mode value M 0 in the RAM of the storage unit 7c, then, control proceeds to the step S17. At the step S17, the front and rear wheel revolution detection values nF, nR are read out of the front and rear wheel revolution sensors 17F, 17R, respectively, and at the subsequent step S18, the revolution difference ΔV W between the front and rear wheels is calculated using the front and rear wheel revolution detection values nF, nR. Then, at the step S19, the front wheel torque distribution command value T 2 is set in accordance with the revolution difference ΔV W between the front and rear wheels calculated at the step S18, and the characteristic as shown in FIG. 10, then, control proceeds to the step S20 where the front wheel torque distribution command value T 2 is output.
Second, switching is possible from the auto four-wheel drive mode to the two-wheel drive mode in a short time, so that when control proceeds from the step S10 to the step S11, the drive mode detection value M (=2) is read out of the drive mode switch 90. At the step S12, due to the drive mode detection value M=2, control proceeds to the step S14. At the step S14, due to the previous drive mode value M 0 =AUTO4, control proceeds to the step S16.
At the step S16, since the elapsed time n 1 ΔT S during switching from the auto four-wheel mode to the two-wheel drive mode is not greater than the switching determination time t 1 , control is continuously executed from the steps S17 to S20, then returns to the main program. And, if the condition at the step S16 is satisfied, i.e. the elapsed time n 1 ΔT S is not greater than the switching determination time t 1 , control from the steps S17 to S20 is repeatedly executed, obtaining the auto four-wheel drive state.
On the other hand, if the elapsed time n 1 ΔT S is equal to or greater than the switching determination time t 1 at the step S16, the front wheel torque distribution command value T 2 is set to zero at the step S22. Then, at the step S20, the front wheel torque distribution command value T 2 is output.
As a result, when carrying out switching from the direct-coupled four-wheel drive mode (LOCK4) to the two-wheel drive mode (2WD) in a short time, the two-wheel drive mode is selected by operation of the changeover switch 87 with the auto four-wheel drive state being continued during a predetermined period of time or the switching determination time t 1 . Thus, referring to FIG. 13, a variation in the clutch pressure P C is decreased gradually as shown in FIG. 13 due to the front wheel torque distribution command value T 2 set in accordance with the revolution difference ΔV W between the front and rear wheels by repeated control from the steps S17 to S20. The clutch pressure P C is lowered gradually in such a way, so that even if the revolution difference between the front and rear wheels is produced by making a turn in the direct-coupled four-wheel drive mode, and even if "unliberated torque" occurs in the two-wheel/four-wheel drive changeover device 60, the friction plate 66b and the friction disc 66d cooperate with each other with slippage to absorb the revolution difference between the front and rear wheels. This reduces "unliberated torque" accumulated in the two-wheel/four-wheel drive changeover device 60, resulting in no occurrence of a shock in the motor vehicle even upon switching from the direct-coupled four-wheel drive mode to the two-wheel drive mode in a short time.
In the first embodiment, the changeover switch 87 is of the rotary type, alternatively, it may be of the lever type on condition that a contact of the auto four-wheel drive mode is positioned between contacts of the two-wheel drive mode and the direct-coupled four-wheel drive mode.
FIGS. 14 to 24 show a second embodiment of the present invention. Referring to FIG. 14, the part-time four-wheel drive motor vehicle is designed to cruise basically in the front-engine rear-drive (FR) mode, and is provided with an engine 210 as a drive source, front and rear wheels 212FL-212RR, a traction transmission train 214 which can change the ratio of a traction distribution between the wheels 212FL-212RR, an oil pressure supply system 216 which serves to supply an oil pressure for controlling a distribution of traction by the traction transmission train 214, and a controller 218 which serves to control the oil pressure supply system 16.
The traction transmission train 214 includes an automatic transmission 220 for shifting traction out of the engine 210 according to the gear ratio selected, and a transfer 222 for dividing traction out of the automatic transmission 220 between the front wheels 212FL, 212FR and the rear wheels or regular driving wheels 212RL, 212RR. The traction transmission train 214 is constructed so that front wheel traction divided by the transfer 222 is transmitted to the front wheels 212FL, 212FR through a front wheel output shaft 224, a front differential gear 226 and a front wheel drive shaft 228, whereas rear wheel traction is transmitted to the rear wheels 212RL, 212RR through a propeller shaft or rear wheel output shaft 230, a rear differential gear 232 and a rear wheel drive shaft 234.
Referring to FIG. 15, the transfer 222 includes in a transfer casing 240 an input shaft 242 and a first output shaft 244 disposed coaxially to butt at each other. The input shaft 242 is rotatably supported to a front casing 240a through a radial bearing 246, whereas the first output shaft 244 is rotatably supported to a rear casing 240b through a radial bearing 248, so that the two shafts 242, 244 enables relative rotation.
Moreover, a second output shaft 254 is rotatably supported to the front and rear casings 240a, 240b in lower portions thereof in parallel to the input shaft 242 and the first output shaft 244 through bearings 250, 252 disposed to the front and rear casings 240a, 240b, respectively. The input shaft 242 is coupled with an output shaft 256 of the automatic transmission 220, and the first output shaft 244 is coupled with the rear wheel output shaft 230, and the second output shaft 254 is coupled with the front wheel output shaft 224.
An auxiliary transmission unit 258 is interposed between the input shaft 242 and the first output shaft 244, whereas and a two-wheel/four-wheel drive changeover device 260 is interposed between the first output shaft 244 and the second output shaft 254.
The auxiliary transmission unit 258 comprises a planetary gear 262, and a dog-clutch-type high/low speed changeover device 264 disposed coaxially to the planetary gear 262.
The planetary gear 262 comprises a sun gear 262a formed on the outer periphery of the input shaft 242, an internal gear 262b fixed inside the front casing 240a, a pinion gear 262c engaged with the sun gear 262a and the internal gear 262b, and a pinion carrier 262d for rotatably supporting the pinion gear 262c.
The high/low speed changeover device 264 comprises a shift sleeve 264b including a cylindrical portion 264a1 having a spline hole 264b, engaged with a spline shank formed on the outer periphery of the first output shaft 244 and a flange portion 264a 2 integrated with the cylindrical portion 264a 1 at a left end thereof and having an external teeth 264b 2 on the outer peripheral face thereof, a high-speed shift gear 264c formed on the outer periphery of the input shaft 242 which is engageable with the spline hole 264b, of the shift sleeve 264b, and a low-speed shift gear 264d formed on the inner periphery of the pinion carrier 262d which is engageable with the external teeth 264b 2 of the shift sleeve 264b.
Referring to FIG. 16, the shift sleeve 264 is constructed so that a fork 264g integrated with a fork rod 264f disposed slidably in the longitudinal direction is engaged with a peripheral groove 264e formed on the outer peripheral face of a right end of the cylindrical portion 264a 1 , the fork rod 264f being coupled with an auxiliary transmission lever which enables, through a linkage, not shown, linear selection of the two-rear-wheel drive high-speed (2H) range, the four-wheel drive high-speed range (4H), the neutral (N) range, and the four-wheel drive low-speed (4L) range arranged in the vicinity of a driver's seat. When selecting the 2H range and the 4H range by the auxiliary transmission lever, the spline hole 264b 1 is engaged with the high-speed shift gear 24c, and moved to a high-speed shift position H wherein traction transmitted to the input shaft 242 is directly transmitted to the first output shaft 244. In this state, when selecting the N range by the auxiliary transmission lever, the spline hole 264b 1 is separated from both of the high-speed shift gear 264c and a four-wheel drive gear 280, and moved to a neutral position N wherein coupling of the input shaft 242 with the first output shaft 244 is released. Moreover, when selecting the 4L range by the auxiliary transmission lever, engagement of the spline hole 264b, with the high-speed shift gear 264c is released as seen in the lower disposition of the shift sleeve 264b in FIG. 16, and the external teeth 264b 2 is engaged with the low-speed shift gear 264d, the spline hole 264b 1 being moved to a low-speed shift position L wherein the spline hole 264b 1 is engaged with the four-wheel drive gear 280 formed to a first sprocket 268 as will be described later.
Returning to FIG. 15, the two-wheel/four-wheel drive changeover device 260 comprises a wet multiple-disc friction clutch (hereafter refer to as "friction clutch") 266 for changing the ratio of a traction distribution between the front and rear wheels, the above first sprocket 268 disposed rotatably to the first output shaft 244, a second sprocket 270 coupled coaxially with the second output shaft 254, and a chain 272 arranged to allow connection between the first and second sprockets 260, 270.
The friction clutch 266 comprises a clutch drum 266a coupled with the first sprocket 268, friction plates 266b spline-coupled with the clutch drum 266a, a clutch hub 266c spline-coupled with the outer periphery of the first input shaft 244, friction discs 266d coupled integrally with the clutch hub 266c, each being disposed between the friction plates 266b, a rotary member 266e rotating together with the first output shaft 244 and moving axially toward the clutch drum 266a to bring the friction plates 266b into contact with the friction discs 266d, a pin 266k coupled integrally with the clutch hub 266c and for engaging the clutch hub 266c with the rotary member 266e, a clutch piston 266g mounted to an inner wall of the rear casing 240b and being movable axially, a thrust bearing 266f for transmitting axial movement of the clutch piston 266g to the rotary member 266e, a cylinder chamber 266h formed between the inner walls of the clutch piston 266g and the rear casing 240b, and a return spring 266j for providing to the rotary member 266e a basing force in the direction of the clutch piston 266g.
When the oil pressure supply system 216 provides a clutch pressure P C to an input port 274 formed through the rear casing 240b which communicates with the cylinder chamber 266h, a pressing force is generated in the cylinder chamber 266h, so that the clutch piston 266g is moved leftward as viewed in FIG. 15, which is transmitted to the rotary member 266e through the thrust bearing 266f. The friction plates 266b and friction discs 266d separated from each other come in contact with each other by movement of the friction discs 266d, producing a clutch engaging force corresponding to the clutch pressure P C due to a frictional force. Thus, a driving force of the first output shaft 244 is transmitted, according to a predetermined ratio of a torque distribution corresponding to a clutch engaging force of the friction clutch 266, to the second output shaft 254 through the first sprocket 268, the chain 272, and the second sprocket 270.
On the other hand, when the clutch pressure P C as supplied is decreased, and the rotary member 266e and the clutch piston 266g are moved rightward as viewed in FIG. 15 by a biasing force of the return spring 266j so that the friction plates 266b and the friction discs 266d are separated from each other, a driving force of the first output shaft 244 is not transmitted to the second output shaft 254.
A four-wheel drive gear 280 is arranged to the first sprocket 268 on the outer periphery thereof on the side of the shift sleeve 264b. When moving the shift sleeve 264b to the low-speed shift position L as described above, the spline hole 264b l and the four-wheel drive gear 280 are engaged with each other to forcibly couple the first output shaft 244 with the second output shaft 254. Thus, the shift sleeve 264b and the four-wheel drive gear 280 constitute a dog clutch for forcibly forming the four-wheel drive state.
Referring to FIG. 16, disposed inside the front casing 240a are a high-speed shift position sensor 286 for sensing that the shift sleeve 264b is slidingly moved up to the high-speed shift position H. A detection signal S H of the high-speed shift position sensor 286 is input to the controller 218.
Referring to FIG. 17, the oil pressure supply system 16 has a circuit structure as shown in FIG. 17, and provides a predetermined clutch pressure P C to the input port 274 of the transfer 222.
The oil pressure supply system 216 has as an oil pressure source a main pump 300 of the normal/reverse rotation type connected directly to and driven by the first output shaft 244, and a sub-pump 304 of the normal rotation type disposed in parallel with the main pump 300 and driven by an electric motor 302 as a power source. The main pump 300 and the sub-pump 304 inhale hydraulic fluid within an oil tank 305 arranged in lower portions of the front and rear casings 240a, 240b through strainers 306a, 308a, and discharge it into ducts 306b, 308b on the discharge side. Connected to a convergent duct 310a which converges the ducts 306b, 308b is an oil element 312 to which a relief passage 316 is connected on the upstream side thereof, i.e. on the side of the main pump 300 and the sub-pump 304, the relief passage 316 having the other end connected to a lubricating system 314. Moreover, a line-pressure regulating valve 318 is connected to the oil element 312 on the downstream side thereof. Connected to ducts 310b, 310c, 310e which branch off from the convergent duct 310a are an electromagnetic selector valve 320, a clutch-pressure regulating valve 322, and a pressure reducing valve 324 on the input side thereof, respectively. Moreover, connected to the clutch-pressure regulating valve 322 on the output side thereof is a pilot selector valve 326 on the input side thereof, which supplies the clutch pressure P C to the transfer 222 when receiving a pilot pressure out of the electromagnetic selector valve 320, whereas connected to the pressure reducing valve 324 on the output side thereof is a duty-control solenoid valve 328 on the input side thereof. Arranged in the oil tank 305 are a temperature sensor 330 for sensing the temperature of hydraulic fluid, a hydraulic switch 332 for detecting a pressure reduced by the line-pressure regulating valve 318, and a pressure switch 334 for detecting the clutch pressure P C output from the pilot selector valve 326, detection signals thereof being output to the controller 18. As for the actual motor vehicles, the oil pressure supply system 216 is arranged inside the transfer 222. The main pump 300 for inhaling hydraulic fluid out of the oil tank 305 is coupled with the first output shaft 244 through first and second gears 336a, 336b as shown in FIG. 15, whereas the sub-pump 304 is coupled with the electric motor 302 mounted to the rear casing 240b on the outside thereof.
Next, referring to FIG. 17, component parts of the oil pressure supply system 216 will be described in detail.
The main pump 300 rotating in the normal direction inhales hydraulic fluid out of the oil tank 305 through the strainer 306a connected to a suction duct 306c at an end thereof, and the sub-pump 304 also inhales hydraulic fluid out of the oil tank 305 through the strainer 308a connected to a suction duct 308c at an end thereof. Check valves 306d, 308d are arranged in the discharge ducts 306b, 308b of the main pump 300 and the sub-pump 304, respectively, and a bypass passage 340 is arranged to allow communication between the discharge duct 306b of the main pump 300 and the discharge duct 308c of the sub-pump 304. The bypass passage 340 comprises a bypass duct 340a and a triple check valve 340b arranged therein, and is constructed so that when the discharge duct 306b becomes in the negative pressure state, the check valve 340b opens to form a communication passage for allowing passage of hydraulic fluid in the direction of a dotted arrow as shown in FIG. 17.
The relief passage 316 connected to the convergent duct 310a on the upstream side of the oil element 312 comprises a relief duct 316a having the other end connected to the lubricating system 314, and a double spring check valve 316b arranged therein. When an oil pressure on the upstream side of the oil element 312 becomes greater than a predetermined value due to clogging produced in a filter of the oil element 312, the check valve 316b opens to form a communication passage for allowing passage of hydraulic fluid in the direction of a dotted arrow as shown in FIG. 17.
The line-pressure regulating valve 318 comprises a pressure regulating valve of the inner pilot and spring type, including a spool arranged slidably in a cylindrical valve housing having an input port 318A connected to the convergent duct 310a, an output port 318 B connected to the lubricating system 314 and inner pilot ports 318 P1 , 318 P2 receiving primary and secondary pressures through stationary orifices, and a return spring 318a for biasing the spool on the one end side. A line pressure P L increased through the main pump 300 or the sub-pump 304 is decreased to a predetermined value by the line-pressure regulating valve 318, which is provided to the electromagnetic selector valve 320, the clutch-pressure regulating valve 322, and the pressure reducing valve 324. Hydraulic fluid flowing out of the output port 318 B upon decreasing is supplied to the lubricating system 314.
The clutch-pressure regulating valve 322 comprises a pressure regulating valve of the inner and outer pilot and spring type, including a spool arranged slidably in a cylindrical valve housing having an input port 322 A connected to the duct 310c, an output port 322 B connected to the pilot selector valve 326, an inner pilot port 322 P1 receiving as a pilot pressure the secondary pressure through a stationary orifice and an outer pilot port 322 P2 receiving a control pressure out of the duty-control solenoid valve 328, and a return spring 322a for biasing the spool on the one end side. The clutch-pressure regulating valve 322 is constructed so that when receiving no pilot control pressure out of the duty-control solenoid valve 328, a communication passage between the input port 322 A and the output port 322 B is closed so as not to output the secondary pressure, whereas when receiving the pilot control pressure out of the duty-control solenoid valve 328, the spool is moved to output from the output port 322 B , as the clutch pressure P C , the secondary pressure corresponding to the pilot control pressure.
The pressure reducing valve 324 comprises a pressure reducing valve of the inner pilot and spring type and with a constant secondary pressure, including a spool arranged slidably in a cylindrical valve housing having an input port 324 A connected to the duct 310e, an output port 324 B connected to the duty-control solenoid valve 328, an inner pilot port 324 P1 receiving as a pilot pressure the secondary pressure out of the output port 324 B through a stationary orifice and a drain port 324 H , and a return spring 324a for biasing the spool on the one end side. When the spool is moved to a predetermined position by the pilot pressure supplied to the inner pilot port 324 P , the primary pressure out of the input port 324 A is supplied, as a control pressure having a predetermined reduced value, to the duty-control solenoid valve 328.
The duty-control solenoid valve 328 has three ports and two positions. This valve includes an input port 328 A connected to the pressure reducing valve 324, a drain port 328 R connected to a drain, an output port 328 B connected to an outer pilot port 322 P2 of the clutch-pressure regulating valve 322, and a return spring 327a. The duty-control solenoid valve 328 is movable between a normal position 328b wherein a spool arranged in the valve allows communication between the output port 328 B and the drain port 328 R , and an operating position 328c wherein the spool allows communication between the input port 328 A and the output port 328 B . When the controller 218 provides to a solenoid 328d an exciting current i 0 with a predetermined duty ratio, the spool is moved from the normal position 328b to the operating position 328c against the return spring 328a during a period of time that the current i 0 is turned on, outputting to the clutch-pressure regulating valve 322 the pilot control pressure corresponding to the duty ratio. Therefore, when the duty-control solenoid valve 328 supplies the control pressure to the outer pilot port 322 P2 , the clutch-pressure regulating valve 322 provides the clutch pressure P C corresponding to the pilot control pressure, so that a clutch engaging force of the friction clutch 266 is controlled according to this, obtaining a distribution of drive torque to the front wheels in accordance with the clutch pressure P C .
The electromagnetic selector valve 320 is of the spring-offset type, and has three ports and two positions. This valve includes an input port 320 A receiving a line pressure, an output port 320 B connected to an outer pilot port 326 P1 of the pilot selector valve 326, and a drain port 320 D . The electromagnetic selector valve 320 is movable between a normal position 320b wherein a spool arranged in the valve closes the input port 320 A and allows communication of the output port 320 B with the drain port 320 D , and an operating position 320c wherein the spool allows communication between the input port 320 A and the output port 320 B and closes the drain port 320 D . When the controller 218 provides to a solenoid 320d an exciting current i 1 , the spool is moved to the operating position 320c against the return spring 320a during a period of time that the current i 1 is turned on, providing the pilot control pressure to the outer pilot port 326 P1 of the pilot selector valve 326. On the other hand, when the exciting current i 1 out of the controller 218 is turned off, the spool is returned to the normal position 320b by a pressing force of the return spring 320a, so that the pilot control pressure being supplied to the outer pilot port 326 P1 is removed through the drain port 320 D .
Referring also to FIG. 18, the pilot selector valve 326 includes a spool 326e arranged slidably in a cylindrical housing 326i having an input port 326 A receiving the secondary pressure out of the clutch-pressure regulating valve 322, an output port 326 B providing the secondary pressure to the transfer 222, an outer pilot port 32